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The Rutherford Model of the Atom: 100 Years Old

Science is all about models. How does this whole model-science thing work? Basically, you have a model for how something works. When more data is collected, the model sometimes has to be changed. This is what is cool about looking at previous models of the atom. You can see how things have changed. Since the Rutherford model of the atom is now 100 years old, I figured it would be a good idea to see how this model came to be, and what happened after that.

Where to start? How about with the Greeks?

Above:

Greek Model of the Atom

Maybe the Greeks did more philosophy than science. They thought about things, but they rarely compared their ideas with experimental evidence. But what did they think about matter? One of the philosophers that was known to think about this (because of course there were others, but maybe they didn’t have a blog) was Democritus. Here is a picture of him.

At the time, Democritus and his buddies were thinking about matter. They wondered what would happen if you kept taking something (like a tree) and breaking into smaller and smaller pieces? Would it always be a piece of a tree? Could you keep breaking it into smaller and smaller pieces? Democritus said that if you keep breaking it down, you would get to a size that could no longer be broken. This would be the indivisible piece. In Greek, atomos = indivisible. Thus, the atom. (I know there is more to the Greeks, but I need a place to start.)

I like to think of this early Greek atomic model as the Lego model. Look at these awesome Lego models.

There are two ways to think about these two objects. If you break the space shuttle down, you get more space shuttle. The Democritus way says that if you break it down, you get Lego blocks (or Lego atoms since you can't break a brick). Notice that I didn't say Legos — that is not an allowable word.

Dalton’s Model

John Dalton was an English scientist born in 1766. He basically agreed with the Democritus model of the atom. However, Dalton took it one step further.

I am not going to go into the experimental evidence for Dalton's model of the atom, but it's good stuff. Let me just state what Dalton said:

Stuff can be broken into elements (the things listed on the periodic table).

Elements are atoms with different masses.

Compounds are a combinations of elements. You know, like water, salt or pizza.

Basically, Dalton just expanded on the Greek idea of the atom. An atom is a small thing, and there are different masses with different properties.

The Plum Pudding Model

J.J. Thomson (aka J.J.) played with cathode rays. These are just beams of electrons (but cathode ray sounds cooler). By having the beam interact with electric and magnetic fields, Thomson was able to determine the mass to charge ratio for an electron. With this charge to mass ratio, he was pretty sure the electron was really really small. If it was that small, it must come from inside the atom.

Thomson was the first to say that atom isn't really an atom (if you go back to the Greek definition saying atom means indivisible). But what does this have to do with plum pudding? Who eats plum pudding anyway? I guess you could think of raisin pudding or maybe chocolate-chip ice cream.

image via Joel Kraut

In this chocolate-chip model, the electrons are the small chocolate chips (and negatively charged). Since the whole atom is neutral, the ice cream must be some positive stuff.

Rutherford Scattering

Ernest Rutherford said one day, "Hey, I think I will shoot some stuff at atoms." I am sure his wife said, "Oh, Ernie" (she probably called him Ernie) "if it makes you happy to play with your little physics stuff, go ahead. I know how much you like it." So he did. He shot some alpha particles (which are really just the nucleus of a helium atom) at some really thin gold foil. Here is a diagram of his experiment.

If you shoot these positive alpha particles at this positive pudding atom, they should mostly bounce off, right? Well, that is not what happened. Rutherford found that most of them went right through the foil. Some of them did bounce back. How could that be if the plumb pudding model was correct? Rutherford's experiment prompted a change in the atomic model. If the positive alpha particles mostly passed through the foil, but some bounced back. And, if they already knew that the electron was small and negative, then the atom must have a small positive nucleus with the electrons around them.

You might think this would be a good place to stop since it's the 100th birthday of this model. But no. We will continue. This is not where the story ends.

Bohr Model

The model proposed by Niels Bohr is the one that you will see in a lot of introductory science texts. There are a lot of good ideas in this model, but it is not the one that agrees with all of the current evidence. The model tries to make a connection between light and atoms.

Suppose you take some light and you let different colors bend different amounts (think rainbow). This way, you could see what colors are present for different light sources. Here are three different light sources.

Maybe the light from the light bulb is what you would expect. These are the colors of the rainbow. However, suppose you took some hydrogen gas and excited it. There would only be certain colors (only certain wavelengths) of light produced. If you shine light through some hydrogen gas, there will be dark bands of light at those same colors.

So, Bohr said that these colors of light in the hydrogen gas correspond to different energy levels the electron in hydrogen can have. And this is the key to the Bohr model — electrons can only be at certain energy levels in the atom. This is crazy (at least it was crazy for its time). Think about a planet orbiting the Sun. It can be at any energy level. In this case, there is a gravitational force attracting the planet which produces orbital motion. This will work anywhere in the solar system.

Early physicist thought of the electron in an atom a lot like a planet orbiting the Sun. The key difference is that the electron (in the Bohr model) orbits due to an electric interaction and not a gravitational interaction. Well, the other difference in the Bohr model is that the electron cannot orbit (if it does orbit, which it doesn't) at any distance and any energy.

The Bohr model depends on a connection between the frequency of light and the energy of the level change. If light of a frequency corresponding to the energy change interacts with the atom, the electron can absorb the light and jump up a level. If an excited electron jumps down a level, it looses energy. The energy the electron loses becomes light with a frequency corresponding to a the change in energy.

The Bohr model can be quite confusing to introductory students, but the important point is that this model agrees with the following evidence.

Electrons are small and negatively charged

Protons are in the nucleus, whitch is small compared to the size of the atom

For a particular element, only certain frequencies (colors) of light are absorbed or emitted.

Schrodinger and Heisenberg Model

There is a key point about the Bohr model that is no longer accepted in current models of the atom. In the Bohr model, the electrons are still thought to orbit the nucleus just like planets orbit the sun. Actually, this is something that we cannot say is true. The problem with atoms and electrons is that we humans expect them to obey the same rules as things like baseballs and planets. Actually, the rules are the same, but baseballs and planets follow the rules of quantum mechanics without us humans even noticing.

It turns out that we ca'’t really say anything about the trajectory or position of electrons in an atom. What we can say is all about probabilities. We can say what regions an electron is likely to be.

The image above shows a "probability map" of where the electron in a hydrogen atom is likely to be found. The different images show how the probability distribution changes for different energy levels.

If you are looking for some more resources on the different models of the atom, check these out: